Calling Cocoa APIs from C

Question

I don't know if this is possible, but having code in plain C, is it possible to call Cocoa API's from it?
Something like #include <cocoa.h>, add the corresponding library and go for it?

Thanks for the help

Answer 1

If you link against the Cocoa or Foundation frameworks, you can use objective-c within your C code. However, if you want to use the normal messaging syntax, you will have to change the file extension from .c to .m so that it is compiled as objective-c. If you keep the .c extension, you will only be able to use the c-style runtime calls to interact with objective-objects (i.e. objc_msgSend, objc_getClass, etc.).

Examples: Within a .m file

void cFunction() {
    [[NSString alloc] init];
}

Within a .c file

void cFunction() {
    void* cls = objc_getClass("NSString");
    void* obj = objc_msgSend(cls, NSSelectorFromString(CFSTR("alloc")));
    obj = objc_msgSend(obj, NSSelectorFromString(CFSTR("init")));
}

If you choose the second method, see the Objective-C Runtime Reference.

Answer 2

Objective-C is a very thin wrapper on top of C. The compiler just translates

 [obj message:argument];

into a C-call

 obj_msgSend(obj,@selector(message:),argument);

and that's it (where @selector(message:) is a magic encoding turning a selector (method name) into a computer-understandable-thingy.)

As such, there is not much difference between Objective-C and C from the compiler's viewpoint. For example, you can compile a pure C program with an Objective-C compiler, with exactly the same result as you would get by compiling with a C compiler.

So, the easiest way to "mix" a bit of Objective-C with C is to use .m extension so that the compiler use Objective-C.

That doesn't make your program suddenly very Objective-C-y. You can keep your program almost pure C, with the extension .m. With .m, you can add a few lines of Objective-C message calls without problems.